12.3.2. Radio Galaxies
Essentially all of the identified radio galaxies show moderate to strong narrow emission lines such as OII at 3727 Å in their optical spectra, and this property has been used to confirm preliminary optical identifications based on the positional agreement of the radio source and the galaxy. Most of the identified galaxies are classed as giant ellipticals which have a surprisingly narrow dispersion in absolute optical magnitude of -20.8 ± 0.6 (H = 100 km/sec/Mpc). Initially most of the identifications were with galaxies which showed some sort of optical peculiarity. For example, M87 (Virgo A) has a well-known jet extending from its nucleus; NGC 5128 (Centaurus A) contains a conspicuous dark band across the galaxy; Cygnus A has a double nucleus with intense emission lines, which were the basis for the idea that the radio emission was the result of collisions between two galaxies. Other identifications were with Seyfert galaxies such as NGC 1086 and NGC 1275.
Although more recently radio galaxies which show no obvious optical peculiarity have been identified, there has been some degree of bias toward accepting identifications with galaxies which show some abnormality. Since the identification process is a very subjective one, involving not only positional coincidence but the size and structure of the radio source, as well as the presence of strong emission lines and optical features such as jets or dust, the connection between these phenomena and the radio emission is not clear unless the difficult task of obtaining optical identifications and redshifts of a complete sample of radio sources is achieved.
Because two of the early radio source identifications were with the Seyfert galaxies NGC 1068 and NGC 1275, it has been widely thought that the Seyfert phenomenon is associated with intense radio emission. In fact, this is not the case, since none of the other well-known Seyfert galaxies show radio emission much greater than the normal spiral galaxies. Moreover, even NGC 1068 has a luminosity of only 1040 ergs/sec - only slightly greater than other spiral galaxies - and the classification of NGC 1275 as a Seyfert is now questioned by many astronomers. Although a few other radio sources have been identified with galaxies that were later classed as Seyferts, it appears in general that the radio luminosity of Seyfert galaxies does not significantly exceed that of normal spirals, and the fraction of Seyfert galaxies which are strong radio sources appears to be comparable with that of giant ellipticals.
Similarly, attempts to detect radio emission from other "peculiar" galaxies such as the Zwicky compact galaxies, Markarian galaxies, or the interacting galaxies cataloged by Arp and Vorontzov-Velyaminov have been for the most part unsuccessful, and contrary to a widely held belief, there is no evidence that these "exotic" galaxies are more likely to be strong radio sources than galaxies chosen at random.
Most Seyfert and Seyfert-type galaxies do contain a relatively weak small source at their nuclei. These nuclei also show surpris ingly strong infrared emission, with an intensity roughly proportional to the radio flux (van der Kruit, 1971).
Redshifts are available for only about 100 of the identified radio galaxies. These have absolute radio luminosities which range from about 1040 to 1045 ergs/sec. In contrast to the radio galaxies which were all identified as galaxies that are coincident with cataloged radio sources, are the so-called normal galaxies. These are the optically bright galaxies from which weak radio emission has been detected as a result of a special search. The absolute radio luminosity of the "normal" galaxies is of the order of 1037 to 1038 ergs/sec, which is comparable to the power radiated from our own Galaxy.
It is not completely clear to what extent the normal galaxies are a separate class or just an extension of the radio galaxy phenomena. Originally there was some evidence of a relative deficiency of sources with intermediate luminosities in the range 1039-40 ergs/sec. However, it now appears that this was largely a manifestation of the different techniques used to investigate "radio" galaxies and "normal" galaxies and that the luminosity function is continuous in the range 1038 < L < 1045 ergs/sec.
The identified quasars are all very strong radio sources with luminosities comparable with the most luminous radio galaxies. Of the quasars which were first located from optical measurements, only a few percent have been detected as radio sources. But considering the large redshifts, the upper limit to the radio luminosity of the undetected quasars is still large, and comparable with that of the strongest radio galaxies.
Searches for radio emission from rich clusters of galaxies have been more fruitful, although it appears that the radio emission originates in the brightest cluster member, and that cluster membership does not affect the probability of radio emission. About 5 to 10% of the giant elliptical galaxies which are the brightest cluster members show detectable radio emission (Rogstad and Ekers, 1969).
There is some evidence that the radio luminosity depends weakly on the optical luminosity for both galaxies and quasars. In the case of galaxies almost all the strong radio sources show bright emission lines.